Question

What is a tosylate group?

Answer 1

A tosylate is the anion of tosylic acid.

This is tosylic acid (or p-toluenesulfonic acid):

so the conjugate base is a tosylate. It is toluene (methylbenzene) with a sulfonate group attached on the 4-position.

Related are tosyl leaving groups because they are large and they tend to have a low `"pKa"` (making them thermodynamically unstable, i.e. making their bonds weak).

A common "tosyl" compound is tosyl chloride.

NOTE: These actually have sulfonyl groups, not to be confused with sulfonate groups.